This invention relates to a cannula utilized in an apparatus for the distention and irrigation of veins in preparation for transplantation within the human body. In particular, it relates to a cannula having an obliquely beveled tip for easy insertion of the cannula into the vein and a sloping shoulder for firm retention of the cannula in the vein.
The use of autogenous vein grafts in arterial bypass surgery has become a well-known procedure in the treatment of degenerative disorders in the coronary circulation system, and the saphenous vein in the leg is the preferred candidate for these grafts.
In the preparation of the vein segment for the grafting procedure, it is necessary to distend the vein segment after removal from the patient's leg in order to overcome spasm and to identify leaky side branches. Typically, this is done by tying off the upstream end of the vein segment and injecting irrigation fluid (typically a saline solution) into the open downstream end manually by means of a syringe. The typical procedure is described in some detail in U.S. Pat. No. 3,958,557 to Sharp, et al.
It has become known in recent years that distention of the veins at pressures in excess of 500 millimeters of mercury (mm Hg) can damage the vascular endothelium, with a resultant premature loss of vein patency subsequent to the bypass implantation. Thus, it has been established that excessive pressurization during distention can ultimately result in premature failure of the grafted vein segment due to thrombosis, subendothelial hyperplasia, or accelerated atherosclerosis.
Consequently, extreme care must be taken in the distention procedure to avoid excessive pressurization. Until recently, the degree of success achieved in this regard depended solely upon the judgment and skill of the operator performing the procedure. However, it was found that even the most skillful practitioner could not readily detect the relatively high pressures (600 to 700 mm Hg) generated by the syringe, because a vein in spasm has a small diameter and a low wall tension (Laplace's Law).
Thus, it has become apparent that some means is necessary for reliably limiting the distention pressure to a value below 500 mm Hg. In addition, recent experiments have demonstrated that distention pressures in the range of 100 to 400 mm Hg are necessary both to overcome spasm in most veins, and to reveal reliably all unsecured side branches in the vein segments involved. In some cases, static pressures of 400 mm Hg have been found to facilitate complete vein dilation, and transient pressures particularly spastic vein segments. The experiments demonstrated that static pressures as high as 500 mm Hg have been used to dilate particularly spastic vein segments. The experiments demonstrated that static pressures in the range of 100 to 400 mm Hg were well tolerated, producing little, if any, noticeable vein damage.
The results of these experiments indicated the need to provide static distention pressures in the range of 100 to 400 mm Hg, with the further need to allow transient pressures up to, but not substantially in excess of, 500 mm Hg. As previously noted, such static pressures allow substantially complete identification of all unsecured side branches. These side branches are preferably secured during the irrigation procedure, to ensure that none are overlooked.
In the prior art, continuous pressure must, typically, be supplied by the syringe. Thus, the person performing the distention procedure is not able, simultaneously, to secure the side branches, and an additional practitioner is necessary to perform this latter operation. Accordingly, it is desirable to have some means for automatically delivering the irrigation fluid, at the desired pressure, without the continued need for the syringe, thereby allowing a single practitioner first to initiate the irrigation, and then to secure the side branches while the irrigation continues under automatically controlled pressure.
The prior art has taught the desirability of injecting the irrigation fluid into the vein through a cannula inserted into the open end of the vein and secured thereto by a temporary ligature. One such cannula is disclosed in the aforementioned patent to Sharp et al. It has become the usual practice to cannulate the open upstream end of the saphenous vein before the downstream end is removed from the patient's leg. The open end of the vein remains cannulated throughout the irrigation and distention operation, until just before implantation of the vein segment as a coronary bypass. In this manner, the cannula provides easy identification of the respective ends of the vein, an important consideration since the valves in the vein allow blood flow in only one direction.
The typical prior art cannula has a blunt, circular leading edge on its tip, which is normally tapered to have an outside diameter at the leading edge which is approximately equal to the undistended inside diameter of the vein. The cannula may also include a peripheral flange to facilitate a temporary fluid-tight ligation of the vein on the cannula. It has been found that the blunt, circular leading edge of such a cannula is very difficult to insert into the vein, making this maneuver tedious and unnecessarily prolonged.